1. Field of the Invention
The present invention generally relates to an apparatus and method for coating print media in an inkjet printer system. More particularly, the present invention relates to an apparatus and method that utilizes motion control to improve coatweight uniformity in intermittent coaters in a printer pre-coating apparatus related to an inkjet printer system.
2. Background Art
Drop-on-demand ink jet printers use thermal energy to produce a vapor bubble in an ink-filled chamber to expel a droplet. A thermal energy generator or heating element, usually a resistor, is located in the chamber on a heater chip near a discharge nozzle. A plurality of chambers, each provided with a single heating element, are provided in the printer""s print head. The print head typically includes the heater chip and a nozzle plate having a plurality of the discharge nozzles formed therein. The print head forms part of an ink jet print cartridge that also has an ink-filled container.
The performance of ink jet printers have typically suffered from two major shortcomings. First, optical density of a printed image varies greatly with the print media or substrate being printed upon. Second, ink drying time sometimes may be excessive on some media types.
The performance of the ink jet printer is affected by interaction between the ink and print media or substrate influences. Different media types interact differently with the ink and not all media types are well suited for ink jet printing. Accordingly, attempts have been made to apply a liquid coating to the media before printing because the liquid coating consistently interacts with the ink no matter what type the printing media is, the quality of the resulting printed image can be improved. The ink may contain, for example, penetrants to improve dry time and binders to improve performance. These xe2x80x9cprecoatingxe2x80x9d liquids may contain materials that cause the ink to flocculate on the surface of the media, improving image quality. Precoating liquids have previously been applied to the print media using a separate ink jet print head and by the use of a roll coating apparatus that directly contacts the print media prior to ink application. One roll coating apparatus and method of the prior art is shown and described in U.S. Pat. No. 6,183,079, assigned to Lexmark International, Inc., which is incorporated herein by reference.
In such a system, as known to people skilled in the art, a fluid coating is applied to a printing medium such as a sheet of paper just prior to printing. Printers having pre-coating system may provide many advantages over conventional printers including the improvement of the optical density (or color saturation) of pigmented inks, reduction of color bleed, improved water-fastness, reduction of cockle and curl in the paper, and improved drying times.
One type of a pre-coating system utilizes a roll coating mechanism. Roll coating mechanisms have a long history and the technology is generally well known. Such systems, however, are almost exclusively used in a continuous mode. Remedies for the non-uniformities and defects which arise from the starting, stopping, and idling during printing are not generally addressed because in a continuous system, they account for an acceptably small part of the total job.
Precoating systems of the prior art, however, suffer from several shortcomings. For example, ink jet precoating systems require that the precoating liquid have a sufficiently low viscosity to pass consistently through the print head. Such liquids typically have an undesirably long dry time and cause undesirable cockle and curl in the medium. Prior art roll coating precoating systems have not provided optimum control over the amount of precoating liquid applied to the print medium. Because the roll coater typically remains in contact with the medium during stop-start printing, coat weight irregularity, often referred to as xe2x80x9cbanding,xe2x80x9d has occurred in prior art roll coating systems. Banding frequently occurs when the rolls are stopped and the printer is depositing ink onto the substrate. During that time, coating remaining on the rolls may be absorbed by the substrate, resulting in a high coat weight at that location and a visible band. Severe banding may be aesthetically unacceptable and may disturb the interaction between the coating liquid and the ink.
One way to overcome the potential coatweight non-uniformity and defects that arise from the starting, stopping, and idling during printing is to utilize a xe2x80x9cpre-spinxe2x80x9d motion. That is, the rolls in a pre-coating system may be turned for some amount of motion time (or some equivalent angle or distance from the idle position) so that the fluid can be redistributed. Note that this motion (xe2x80x9cpre-spinxe2x80x9d) would occur before the page is staged in the nip between the applicator and back-up rolls and so no paper would be in the system during this time. After the redistribution has occurred, the paper can then be staged in the nip between the applicator and back-up rolls and coating can begin. While this xe2x80x9cpre-spinxe2x80x9d eliminates most of the non-uniformities which occur as a result of the aforementioned flow of coating fluid, it causes another problem. The problem is that the coatweight on each of the rolls in the system will tend toward a different amount when the system is run without paper present than it would when there is paper present. If this xe2x80x9cpre-spinxe2x80x9d motion of the coater is too long, then other non-uniformities will occur on the coated page because the system equilibrates to different coatweights on its rolls when it runs without paper than when it runs with paper.
Accordingly, there is a need for an improved ink jet printer that is capable of printing images uniformly on a wide variety of commercially available substrates and wherein ink drying time is minimized and printed image quality is maximized.
The present invention, in one aspect, is a coating apparatus for applying a coating liquid to a printing substrate from a first paper path. The apparatus includes a rotatable first roll, and a rotatable second roll positioned adjacent to the first roll and defining with the first roll a first nip through which the printing substrate passes. The apparatus also has a metering device for applying a layer of coating liquid onto the second roll, which in turn transfers the coating liquid to the printing substrate. A controller communicates with at least the second roll, wherein the controller performs the steps, of determining whether the idle time of the second roll is longer than a predetermined threshold, setting a pre-spin flag if the idle time of the second roll is longer than a predetermined threshold, and directing the second roll to perform a pre-spin upon the presence of the pre-spin flag.
In one embodiment, the metering device includes a supply of coating liquid in contact with the second roll, and a doctor blade contacting the second roll for metering a layer of coating liquid onto the second roll. In another embodiment, the metering device includes a rotatable third roll contacting the second roll and forming a second nip therebetween, a supply of coating liquid in contact with the third roll, and a doctor blade contacting the third roll.
The coating apparatus can be associated with a printer. The controller thus can perform the steps of determining whether the printer is in a stand-by state, and directing the second roll to perform a pre-spin if the printer is in a stand-by state. The printer can have a second paper path to allow the printing substrate to bypass the first paper path. Accordingly, the controller further performs the steps of determining whether the printing substrate is in the first paper path, and directing the second roll to perform a pre-spin if the printing substrate is in the first paper path. Moreover, the controller further performs the steps of determining whether the printing substrate is in the first paper path, determining whether the printer is in a stand-by state, and directing the second roll to perform a pre-spin if the printing substrate is in the first paper path and the printer is in a stand-by state. In operation, the second roll performs the pre-spin at an optimal rotating angle to optimize the coatweight uniformity of the coating liquid to the printing substrate, wherein the optimal rotating angle is substantially in the range of 360 to 720 degrees. Optionally, the apparatus may have a timer coupled to the controller. In one embodiment, the predetermined threshold is substantially equal to five (5) minutes.
In another aspect, the invention relates to a method for applying a coating liquid to a printing substrate. The method includes the steps of providing a coating device having a rotatable first roll, a rotatable second roll positioned to the first roll and defining with the first roll a first nip which the printing substrate passes, and a metering device for applying a layer of coating liquid to the printing substrate, determining whether the idle time of the second roll is longer than a predetermined threshold, setting a pre-spin flag if the idle time of the second roll is longer than a predetermined threshold, and directing the second roll to perform a pre-spin upon the presence of the pre-spin flag. The method further includes the step of applying a layer of coating liquid to the printing substrate.
In one embodiment, the coating apparatus is associated with a printer, the method includes the steps of determining whether the printer is in a stand-by state, and directing the second roll to perform a pre-spin if the printer is in a stand-by state.
In another embodiment, the coating device is associated with a printer, the printer having a first paper path and a second paper path to allow the printing substrate to bypass the first paper path, the method includes the steps of determining whether the printing substrate is in the first paper path, and directing the second roll to perform a pre-spin if the printing substrate is in the first paper path. The method further includes the steps of determining whether the printing substrate is in the first paper path, determining whether the printer is in a stand-by state, and directing the second roll to perform a pre-spin if the printing substrate is in the first paper path and the printer is in a stand-by state. The second roll performs the pre-spin at an optimal rotating angle to optimize the coatweight uniformity of the coating liquid applied to the printing substrate, wherein the optimal rotating angle is substantially in the range of 360 to 720 degrees.
In yet another aspect, the present invention relates to an apparatus for applying a coating liquid to a printing substrate from a first paper path. The apparatus has an applicator roll for applying the coating liquid to the printing substrate, and a controller means communicating with the applicator roll. The controller means performs the steps of determining whether the idle time of the applicator roll is longer than a predetermined threshold, setting a pre-spin flag if the idle time of the applicator roll is longer than a predetermined threshold, and directing the second roll to perform a pre-spin upon the presence of the pre-spin flag. In one embodiment, the apparatus is associated with a printer, and the controller means further performs the steps of determining whether the printer is in a stand-by state, and directing the applicator roll to perform a pre-spin if the printer is in a stand-by state. The printer may have a second paper path to allow the printing substrate to bypass the first paper path, and the controller means further performs the steps of determining whether the printing substrate is in the first paper path, and directing the applicator roll to perform a pre-spin if the printing substrate is in the first paper path. The controller means may further perform the steps of determining whether the printing substrate is in the first paper path, determining whether the printer is in a stand-by state, and directing the applicator roll to perform a pre-spin if the printing substrate is in the first paper path and the printer is in a stand-by state.
These and other aspects will become apparent from the following description of various embodiments taken in conjunction with the following drawings, although variations and modifications therein may be affected without departing from the spirit and scope of the novel concepts of the disclosure.